The study is a continuation of work already in progress. The effects of sympatomimetic amines without and with alpha and beta blocking agents and after pretreatment with atropine and cooling has recently been studied in the chick embryo of 6 days incubation age. To this purpose a branch of the chorio-allantoic artery is cannulated employing a glass micropipette connected by means of a short length of non-distensible polyethylene tubing to a Statham P23De transducer. The arterial pressure, after suitable amplification, is recorded by means of a Honeywell Model 1508 Visicorder. In this manner, it is possible to continually measure and record the arterial pressure of embryos of various ages. Similar studies employing younger embryos of about 3 1/2 days incubation age have recently begun. It is hoped that from these studies we might learn at which time in development the autonomic nervous system governing the cardiovascular system becomes functional and also at which time in development alpha and beta adrenergic receptors become functional.